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Longman Dictionary of Contemporary English
light pen
noun
EXAMPLES FROM CORPUS
▪ And when the program is ready, the student enters it simply by moving a light pen across the bar codes.
▪ When the light pen is moved over the code, it triggers the chips to make sound through the loudspeaker.
Wiktionary
light pen

n. (context computing English) A pen-like device allowing a computer user to interact with the screen display.

WordNet
light pen

n. (computer science) a pointer that when pointed at a computer display senses whether or not the spot is illuminated [syn: electronic stylus]

Wikipedia
Light pen

A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT display.

It allows the user to point to displayed objects or draw on the screen in a similar way to a touchscreen but with greater positional accuracy. A light pen can work with any CRT-based display and other display technologies, but its ability to be used with LCDs was unclear (though Toshiba and Hitachi displayed a similar idea at the "Display 2006" show in Japan).

A light pen detects a change of brightness of nearby screen pixels when scanned by cathode ray tube electron beam and communicates the timing of this event to the computer. Since a CRT scans the entire screen one pixel at a time, the computer can keep track of the expected time of scanning various locations on screen by the beam and infer the pen's position from the latest timestamp.